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<title>GnuTLS 3.4.8: SRP credentials</title>
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<a name="SRP-credentials"></a>
<div class="header">
<p>
Next: <a href="PSK-credentials.html#PSK-credentials" accesskey="n" rel="next">PSK credentials</a>, Previous: <a href="Certificate-credentials.html#Certificate-credentials" accesskey="p" rel="prev">Certificate credentials</a>, Up: <a href="Associating-the-credentials.html#Associating-the-credentials" accesskey="u" rel="up">Associating the credentials</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="SRP"></a>
<h4 class="subsection">6.4.2 SRP</h4>
<p>The initialization functions in SRP credentials differ between
client and server.
Clients supporting <acronym>SRP</acronym> should set the username and password
prior to connection, to the credentials structure.
Alternatively <a href="Core-TLS-API.html#gnutls_005fsrp_005fset_005fclient_005fcredentials_005ffunction">gnutls_srp_set_client_credentials_function</a>
may be used instead, to specify a callback function that should return the
SRP username and password.
The callback is called once during the <acronym>TLS</acronym> handshake.
</p>
<dl compact="compact">
<dt><code><var>int</var> <a href="Core-TLS-API.html#gnutls_005fsrp_005fallocate_005fserver_005fcredentials">gnutls_srp_allocate_server_credentials</a> (gnutls_srp_server_credentials_t * <var>sc</var>)</code></dt>
<dt><code><var>int</var> <a href="Core-TLS-API.html#gnutls_005fsrp_005fallocate_005fclient_005fcredentials">gnutls_srp_allocate_client_credentials</a> (gnutls_srp_client_credentials_t * <var>sc</var>)</code></dt>
<dt><code><var>void</var> <a href="Core-TLS-API.html#gnutls_005fsrp_005ffree_005fserver_005fcredentials">gnutls_srp_free_server_credentials</a> (gnutls_srp_server_credentials_t <var>sc</var>)</code></dt>
<dt><code><var>void</var> <a href="Core-TLS-API.html#gnutls_005fsrp_005ffree_005fclient_005fcredentials">gnutls_srp_free_client_credentials</a> (gnutls_srp_client_credentials_t <var>sc</var>)</code></dt>
<dt><code><var>int</var> <a href="Core-TLS-API.html#gnutls_005fsrp_005fset_005fclient_005fcredentials">gnutls_srp_set_client_credentials</a> (gnutls_srp_client_credentials_t <var>res</var>, const char * <var>username</var>, const char * <var>password</var>)</code></dt>
</dl>
<dl>
<dt><a name="index-gnutls_005fsrp_005fset_005fclient_005fcredentials_005ffunction"></a>Function: <em>void</em> <strong>gnutls_srp_set_client_credentials_function</strong> <em>(gnutls_srp_client_credentials_t <var>cred</var>, gnutls_srp_client_credentials_function * <var>func</var>)</em></dt>
<dd><p><var>cred</var>: is a <code>gnutls_srp_server_credentials_t</code> type.
</p>
<p><var>func</var>: is the callback function
</p>
<p>This function can be used to set a callback to retrieve the
username and password for client SRP authentication. The
callback’s function form is:
</p>
<p>int (*callback)(gnutls_session_t, char** username, char**password);
</p>
<p>The <code>username</code> and <code>password</code> must be allocated using
<code>gnutls_malloc()</code> . <code>username</code> and <code>password</code> should be ASCII strings
or UTF-8 strings prepared using the "SASLprep" profile of
"stringprep".
</p>
<p>The callback function will be called once per handshake before the
initial hello message is sent.
</p>
<p>The callback should not return a negative error code the second
time called, since the handshake procedure will be aborted.
</p>
<p>The callback function should return 0 on success.
-1 indicates an error.
</p></dd></dl>
<p>In server side the default behavior of <acronym>GnuTLS</acronym> is to read
the usernames and <acronym>SRP</acronym> verifiers from password files. These
password file format is compatible the with the <em>Stanford srp libraries</em>
format. If a different password file format is to be used, then
<a href="Core-TLS-API.html#gnutls_005fsrp_005fset_005fserver_005fcredentials_005ffunction">gnutls_srp_set_server_credentials_function</a> should be called,
to set an appropriate callback.
</p>
<dl>
<dt><a name="index-gnutls_005fsrp_005fset_005fserver_005fcredentials_005ffile"></a>Function: <em>int</em> <strong>gnutls_srp_set_server_credentials_file</strong> <em>(gnutls_srp_server_credentials_t <var>res</var>, const char * <var>password_file</var>, const char * <var>password_conf_file</var>)</em></dt>
<dd><p><var>res</var>: is a <code>gnutls_srp_server_credentials_t</code> type.
</p>
<p><var>password_file</var>: is the SRP password file (tpasswd)
</p>
<p><var>password_conf_file</var>: is the SRP password conf file (tpasswd.conf)
</p>
<p>This function sets the password files, in a
<code>gnutls_srp_server_credentials_t</code> type. Those password files
hold usernames and verifiers and will be used for SRP
authentication.
</p>
<p><strong>Returns:</strong> On success, <code>GNUTLS_E_SUCCESS</code> (0) is returned, or an
error code.
</p></dd></dl>
<dl>
<dt><a name="index-gnutls_005fsrp_005fset_005fserver_005fcredentials_005ffunction"></a>Function: <em>void</em> <strong>gnutls_srp_set_server_credentials_function</strong> <em>(gnutls_srp_server_credentials_t <var>cred</var>, gnutls_srp_server_credentials_function * <var>func</var>)</em></dt>
<dd><p><var>cred</var>: is a <code>gnutls_srp_server_credentials_t</code> type.
</p>
<p><var>func</var>: is the callback function
</p>
<p>This function can be used to set a callback to retrieve the user’s
SRP credentials. The callback’s function form is:
</p>
<p>int (*callback)(gnutls_session_t, const char* username,
gnutls_datum_t *salt, gnutls_datum_t *verifier, gnutls_datum_t *generator,
gnutls_datum_t *prime);
</p>
<p><code>username</code> contains the actual username.
The <code>salt</code> , <code>verifier</code> , <code>generator</code> and <code>prime</code> must be filled
in using the <code>gnutls_malloc()</code> . For convenience <code>prime</code> and <code>generator</code> may also be one of the static parameters defined in gnutls.h.
</p>
<p>Initially, the data field is NULL in every <code>gnutls_datum_t</code>
structure that the callback has to fill in. When the
callback is done GnuTLS deallocates all of those buffers
which are non-NULL, regardless of the return value.
</p>
<p>In order to prevent attackers from guessing valid usernames,
if a user does not exist, g and n values should be filled in
using a random user’s parameters. In that case the callback must
return the special value (1).
See <code>gnutls_srp_set_server_fake_salt_seed</code> too.
If this is not required for your application, return a negative
number from the callback to abort the handshake.
</p>
<p>The callback function will only be called once per handshake.
The callback function should return 0 on success, while
-1 indicates an error.
</p></dd></dl>
<hr>
<div class="header">
<p>
Next: <a href="PSK-credentials.html#PSK-credentials" accesskey="n" rel="next">PSK credentials</a>, Previous: <a href="Certificate-credentials.html#Certificate-credentials" accesskey="p" rel="prev">Certificate credentials</a>, Up: <a href="Associating-the-credentials.html#Associating-the-credentials" accesskey="u" rel="up">Associating the credentials</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
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